The previous work from this laboratory described progress in the investigation of the disease "ketotic hyperglycinemia" and its effects are glycine metabolism and transport. In this year's report are described inhibition of glycine oxidation of the interconversion of glycine and serine by products of isoleucine catabolism. Also described are the selection of fibroblast cell lines with mutant isoleucine transport systems by use of a selective medium. The studies of glycine metabolism indicate that the hyperglycinemia seen in "ketotic hyperglycinemia" is a secondary phenomenon. The studies of isoleucine transport suggest an approach for the selection of cells with mutant transport systems and offer a tool for the further study of amino acid transport in cultured human fibroblasts.